Magnetic resonance imaging (hereinafter sometimes abbreviated as MRI), that is, an imaging method based on the principle of nuclear magnetic resonance (NMR), is widely employed in medical settings because this method is only mildly invasive, has high spatial resolution and is excellent for examining morphologies. In general, MRI images (not spectra) are obtained based on a difference in relaxation time and are generically used because of the problem of background noise caused by the approximately 60% of water and lipid in a living body.
In general, 1H NMR, 1H is irradiated with pulses, so as to detect an NMR signal therefrom. In contrast, in multiple resonance NMR, an NMR signal is detected by utilizing magnetization coherence transfer of the NMR signal of 1H to an adjacent NMR active nucleus, and by this method, a specific chemical bond, such as a 1H—13C, a 1H—13C—15N sequence or a 1H—13C—13C sequence, can be selectively detected. Here, 13C and 15N are stable isotopes of 12C and 14N, respectively.
If a 1H—13C sequence is to be selectively detected, the magnetization transfers from 1H to 13C and then back to 1H, and thus, the proton in the 1H—13C sequence can be detected. Alternatively, if a 1H—13C—15N sequence is to be selectively detected, the magnetization transfers from 1H through 13C to 15N and then back through 13C to 1H, and thus, the proton in the 1H—13C—15N sequence can be detected. This method is largely characterized by low natural abundances of these sequences. For example, the natural abundance of 1H—13C—15N is as low as 0.0040% (because the natural abundances of 13C and 15N are 1.1% and 0.37%, respectively), and therefore, background noise that causes a problem in conventional magnetic resonance imaging is largely suppressed. Consequently, a compound containing a sequence of 1H—13C—15N or the like shows high selectivity and high sensitivity as a contrast agent for nuclear magnetic resonance analysis or magnetic resonance imaging.
PTL 1 discloses the following: A choline chloride labeled with 13C and 15N is administered to a tumor-bearing mouse through a tail vein. The liver, kidney and tumor extracted from the mouse 1 h after the administration are ground, and the resultant mixture is subjected to centrifugal separation for removing impurities. When the thus obtained supernatant solution is analyzed by 1H—{13C—15N} triple resonance NMR, a 1H signal derived from a methyl group of choline at 3.0 ppm can be detected.